The invention relates to filters made of nonwoven fabric, paper or the like for gaseous media, having an electrically conductive coating made of a conductive substance of pulverizable materials.
The use of filters for removing dust from gaseous media is generally widespread. Thus, surface filters are utilized in various formats, for example, as bag filters, filter cartridges or filter plates. The filtering layer is very frequently formed from nonwoven fabric or paper. The effectiveness of such a filter is influenced by the cake of dust forming on the untreated air side. As the cake of dust increases on the surface of the filter, a higher separation of dust is achieved. At the same time, however, permeability of the filter to air is reduced, and the pressure difference of the filter increases. When the pressure difference exceeds a predetermined limiting value, it is necessary to clean the dust cake and the embedded dust from the filtering layer. This cleaning is usually effected by a pressure surge in the reverse direction of the direction of flow during the filter operation.
For certain filter utilization purposes, the filtering layers can be furnished with electrically conductive substances. Thus, for example, only antistatic and grounded filter elements are permissible for the extraction of explosive dust. This prevents sparks which are produced electrostatically from triggering an explosion. Consequently, the filter components which are electrostatically non-conductive per se must be provided with an antistatic finish. For example, it is known that the filtering layer is provided with a metallic coating or even a coating made of conductive carbon-black particles. For instance, the metallic coating can be applied to the filtering layer by vapor-deposition. In this case, the required surface resistance is already achieved with very thin coatings. The porosity and the air permeability of the filtering medium are scarcely influenced. However, the metallic coatings are very unstable with respect to abrasive stress or the effect of chemicals. Therefore, their application range is highly restricted. In addition, in the customary vapor-deposition methods, only the top surface of the filter medium is provided with a metallic coating, so that the volume resistance assumes values which are too high when working with dense or thicker materials.
It may be that, in comparison to the metallic layer, the full-surface impregnation of the filter with conductive carbon black and a suitable binding-agent system has the advantage of good resistance to abrasion and chemicals; however, filtering properties such as air permeability or porosity of the filter medium are significantly reduced when working with this finishing. Thus, the serviceability of the filter for removing dust is limited. The energy consumption for the operation of the filter is increased and the service life is reduced. In addition, in the case of many dusts, this finishing increases the adherence to the filtering medium to the extent that the ability for regeneration is altered negatively or no longer exists at all.
The object of the present invention is to produce a filter from nonwoven fabric, paper or the like for gaseous media which exhibits high serviceability. The intention is for the abrasive stress to be as low as possible and the resistance to chemicals to be as high as possible. The filter should be capable of regeneration and have a long service life. There is also a striving for high porosity and high air permeability. A primary goal is the antistatic finish, so that the filter can even be used when cleaning gases having explosive dust.
These and other objects of the invention are achieved by the invention which provides a filter made of nonwoven fabric, paper or the like for gaseous media, having an electrically conductive coating made of a conductive substance of pulverizable materials, in particular from electrically conductive carbon-black particles and/or powdery metal particles or other pulverized materials, the carbon-black particles or metal particles being fixed to the filter fibers with the aid of binding agents, and the conductive coating being applied in a reticulated manner on the filtering layer.